The WM59 monoclonal antibody reacts with human CD31, also known as platelet-endothelial cell adhesion molecule-1 (PECAM-1) and gpIIa. This 130-140kDa surface protein is expressed by endothelial cells and at low levels on leukocytes and platelets. It has been reported that CD38 binds to CD31. Homotypic interaction of CD31 is important in adhesion, cell-cell and cell-matrix interaction, and signal transduction.